Put all ingredients together in a large saucepan. Bring to the boil on the stove and simmer gently for 2 hours, stirring occasionally, until the sauce has reduced by half.
Remove the saucepan from the heat and pour the sauce through a sieve into a bowl. Let the sauce cool before serving.
Makes about 1 liter of sauce. Leftovers can be kept in a bottle or container in the refrigerator, and the sauce can also be frozen.